We intend to continue studies in inbred mice to examine the nature of CTLs and CTL precursors that mediate inhibition of the growth of MuLV-induced syngeneic tumors and the availability of effector cell precursors in mice with progressively growing tumors. Additional experiments will explore the role of host T cells in modifying the in vivo antitumor activity of passively transferred immune T cells and the role of purified helper and suppressor T cells and macrophages in regulating the in vitro induction of antitumor CTL responses. The role of each cell type will be defined first in normal and/or immune mice and then examined in mice with progressively growing tumors. These studies should clarify our understanding of host cellular immune mechanisms which regulate antitumor effector responses; they may provide a rational basis for inducing antitumor effector activity by otherwise unresponsive cells from mice with growing tumors. (LB)